An ink jet printer that uses a shear-mode ink jet head is known. The shear-mode inkjet head has a two-layer structure in which two piezoelectric members polarized in a mutually opposite direction in a direction of the thickness are pasted via an adhesive, the two piezoelectric members are cut so that multiple grooves pass pasted faces at a fixed interval, a comb structure that the end of each groove is open at the front end of a plate is formed, and the upside of these grooves is closed by another plate. The rear end of each groove communicates with a common ink chamber and an orifice plate having an ink jet (a nozzle) in a position of each groove is provided to an opening at the end. An electrode is provided in the groove.
FIG. 7 is an explanatory drawing for explaining the structure of such a shear-mode ink jet head and a case of a split drive (in FIG. 7, a three-split drive is shown).
FIG. 7 is a schematic drawing showing a pressure chamber corresponding to each nozzle on a plane perpendicular to an ink jet direction, an upper half and a lower half of a wall of each groove functioning as the pressure chamber form two-layer structure in which two piezoelectric members polarized in a mutually opposite direction in a direction of the thickness are pasted, the upper half and the lower half are covered with each plate, and each groove forms the pressure chamber isolated from an adjacent groove.
Referring to FIG. 7, the split drive (in FIG. 7, the three-split drive is shown) of such a shear-mode ink jet head will be described by comparing nozzles that jet at the same time (nozzles A0 to C3 which are shown in FIG. 7 and each of which corresponds to the pressure chamber over ink surrounded by the piezoelectric members) below.
As the shear-mode inkjet head shown in FIG. 7 is driven with it split into three), each nozzle and each pressure chamber are split into three sets of A, B, C. Nozzles of each set are shown in a state in which suffix numbers 0 to 3 showing numbers of the nozzles that belong to each set (in this example, 0 to 3 are added for convenience sake of the drawing and the description) are added. The wall made of piezoelectric material for partitioning the pressure chamber corresponding to each nozzle is shown as AB0, BC0, - - - , BC1, CA2, - - - for example using alphabetical letters showing sets of nozzles adjacent to the wall and the corresponding suffix number. However, in FIG. 7, suffix numbers out of codes showing the walls are collectively shown over a brace over the alphabetical letters.
In FIG. 7, when no ink is jetted from a nozzle A0 and ink is to be jetted from nozzles A1 to A3, the wall on the left side (CAn−1) of each nozzle for ink to be jetted is displaced leftwards, the wall on the right side (ABn) is displaced rightwards, and the corresponding pressure chamber is expanded. As the volume of the ink chamber of the above-mentioned each nozzle is reduced when the displaced walls are restored, ink is jetted from each nozzle.
For an ink jet apparatus utilizing the above-mentioned shear-mode ink jet head, the one disclosed in JP-A-2000-135787 is known.
According to the shear-mode ink jet head, nozzles at both ends out of multiple nozzles arranged in a main scanning direction have a tendency that the quantity of jetted ink is unstable because the nozzles at both ends are different in a condition from inside nozzles surrounded by each nozzle at both ends, the quantity of jetted ink increases at the end of an image and the density increases.
Then, as for an ink jet recording head configured so that plural ink chambers partitioned by partition walls made of piezoelectric material are arranged and ink is jetted from each ink chamber by applying a driving signal to a driving electrode of the partition wall and deforming the partition wall, there is also proposed an idea for solving the above-mentioned problem that a condition of jetting ink for all ink chambers is equalized by making the outside ink chamber a dummy ink chamber and also filling the dummy ink chamber with ink.
However, as no ink chamber exists outside the outermost dummy ink chamber, pressure in an ink chamber inside the outermost dummy ink chamber gets away because of the deformation of an external wall when it is inevitable that the external wall of the dummy ink chamber is deformed because the external wall has only the same thickness as that of each partition wall, and finally, a difference is made between an ink jet characteristic of the inside ink chamber and an ink jet characteristic of the other ink chamber. As a result, it is difficult to completely solve a phenomenon that the quantity of jetted ink increases at the end of an image and the density increases by the above-mentioned shear-mode ink jet head.
In addition, there is another problem that when plural shear-mode ink jet heads described above are used with them arranged, the quantity of jetted ink increases in a boundary between head modules, printing density increases and striped density nonuniformity emerges in a part equivalent to the boundary. Further, in configuration like a line printer, plural heads are arranged in line, however, as adjacent heads are arranged off in a sub-scanning direction so that nozzles for a few dots are overlapped in the sub-scanning direction, the configuration has a problem that the number of dots overlapped at the ends of the heads further increases and density nonuniformity in a boundary between head modules becomes more conspicuous.
FIG. 8 shows the ink jet head disclosed in JP-A-2000-135787 and the quantity of ink jetted from the dummy ink chamber can be matched with that of other ink chambers by providing the dummy ink chamber at the end of the head and driving the dummy ink chamber when ink is jetted from the effective ink chamber on the side of the end of the nozzle. Plural nozzles are provided to the dummy ink chamber, however, no ink is jetted from them. However, in the invention, as shown in FIG. 8A, the ends of adjacent heads can be overlapped in the sub-scanning direction, but a change of joined positions shown in FIG. 8B is impossible.